Local Area Networks (LANs) have been in existence for over 30 years. They are widely used in universities, enterprises and government offices. While LANs in the past have operated over wired media, lately we have been witnessing a boom in the deployment and usage of Wireless Local Area Networks (WLANs). Once only seen within the enterprise, WLANs are increasingly making their way into residential, commercial, industrial and public deployments. Recent efforts by telephone carriers to integrate LANs and WLANs into their wide-area service offerings are testimony to their growing role in the future of networking.
Voice continues to be the most predominant wireless application and in order to fully integrate with existing and future cellular systems both LANs and WLANs must deliver high quality voice service. In addition to voice services, carriers plan to offer other services such as video conferencing and data services such as Internet access.
The most common LAN protocol is Ethernet (IEEE802.3). The Ethernet protocol was designed to enable fair sharing of a common medium among several nodes. The basic access scheme is called Carrier Sense Multiple Access with Collision Detection (CSMA/CD).
In CSMA/CD, each node with data to send must first listen to the channel to make sure that there is no other node transmitting. This action is called carrier sensing. If the channel is determined to be free, the node can then begin transmitting. Once a node starts transmitting, all other nodes wait until the channel becomes free again before trying to transmit. Since signals take a finite amount of time to travel from one end of an Ethernet segment to the other, two or more nodes can determine the channel to be free and start transmitting simultaneously. When several nodes transmit at the same time a collision occurs. All nodes can detect the collision and halt their transmission.
Following a collision, the involved nodes choose a random back off period before trying to transmit their Ethernet frames again. If repeated collisions occur for a given transmission attempt, the node expands the interval from which the back off time is selected according to an algorithm known as truncated binary exponential back off algorithm. A retransmission is aborted after 16 unsuccessful attempts.
It is intuitive that as the number of nodes on a given Ethernet segment increases, and as the traffic level increases, more collisions occur as part of the normal operation of an Ethernet. The Ethernet protocol is designed to provide fair access to a shared channel.
That is all nodes within the same Ethernet have the same priority. Hence Ethernet provides a best effort data delivery system. While the best effort design of the Ethernet protocol incurs low complexity and implementation cost, it is not capable of providing different levels of service based on users' and applications' needs.
Several schemes were developed to enhance the provided quality of service in an Ethernet environment. Subnet Bandwidth Manager is one of the existing solutions that was introduced in IETF RFC 2814. The basic concept is as follows: A Subnet Bandwidth Manager (SBM) is placed in each subnet. Users that require a better than best effort quality of service use the RSVP signaling protocol to communicate with the SBM to reserve bandwidth. SBM accepts new requests only if resources are available for the new flow. Best effort users are automatically admitted into the subnet without any admission control. In the absence of any traffic control or priority mechanisms, the SBM-based admission control mechanism only limits the total traffic load. Thus, no separation of traffic flow occurs to protect flows requiring a higher level of QoS from the best-effort traffic on the same media. Therefore, given the lack of control of best effort traffic and depending on the traffic flow load, the network might not honor reservations made by flows requiring a higher level of QoS over reservations made by best effort traffic flows.
The most widely spread WLAN protocol is IEEE802.11. IEEE802.11 was designed to provide channel sharing similar to Ethernet (IEEE802.3) in a wireless environment. IEEE802.11 has two different modes of operation. The first is the basic mode called Distributed Coordination Function (DCF). DCF is a contention-based access mode that adopts the Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) scheme. The second, an optional mode called Point Coordination Function (PCF), uses a polling access strategy.
The basic DCF mode is based on Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA). CSMA/CA is a contention mode access method, where all stations, with data to send, contend for the channel. The channel is awarded randomly to a contending station with no regards to the QoS requirements of any of the contending stations. The contention is managed as follows. Before transmitting, each station with data to send must first sense the medium to make sure that it is free. Once the medium is judged to be free, for a period of time equal to one Distributed Inter Frame Space (DIFS), the station randomly picks a number between 1 and its current congestion window (minimum congestion window is 31. After each collision the congestion window is doubled until it reaches a maximum). The station then starts counting down. The station that reaches zero first starts its transmission. Once a station starts transmitting, all other stations freeze their counters until a new DIFS amount of idle time is observed. After each successful transmission, the receiving station is allowed to send an acknowledgment back to the sender. A packet is perceived lost and is retransmitted if an acknowledgment does not arrive at the sender within a timeout period. Obviously this mode (DCF) does not support any quality of service assurance since there is no assurance that any station may get the channel to transmit its packets.
Since DCF is the basic mode, it can operate in a stand-alone fashion. PCF on the other hand is optional and can only exist in conjunction with the DCF mode of operation. When PCF is supported, time is divided into frames. Each frame is further divided into two sub-frames. One sub-frame supports the basic DCF mode where stations contend for the channel. The second sub-frame supports a reservation and polling based PCF medium access. Stations that need to participate in the PCF mode register with the access point. Registered stations are then polled, during the PCF period, to determine if they have data to send. However, there is no guarantee that all registered stations get polled during each PCF sub-frame. The number of polled stations in each PCF period depends on the number of stations requesting the PCF mode as well as the number of stations using the DCF mode. In the PCF mode, the frame size is fixed a priori. The DCF mode is guaranteed a minimum share of this frame. However, the PCF mode has no explicit guaranteed minimum share of the frame. Therefore, depending on the frame size, the number of users requiring PCF mode and the number of users in the DCF mode, the PCF mode may not provide the required QoS to it users. Several studies have shown that neither DCF Several studies have shown that neither DCF nor PCF modes can provide adequate quality of service to its users.
Similar to the Ethernet case, several solutions were designed to enhance the quality of service in IEEE802.11. Wireless Rether was an offshoot of work done on the Rether protocol in a wired environment. Wireless Rether adopts a centralized token exchange scheme where each host in the LAN needs to receive a token from a Wireless Rether Server before it may transmit data. Wireless Rether attempts to guarantee bandwidth to individual applications and may require modifications to each host. In this scheme, a host may only communicate and send data traffic if the token is obtained and registration of the host is completed. Further, the total bandwidth reserved varies on the length of time the token is held. After data transmission, this token needs to be passed back to the server. Depending on the configuration, this interrupt overhead associated with the passing of the token network access latency may constitute a significant percentage of the cycle time. This problem is compounded if wireless rether is performed over multiple segments to introduce additional communication overhead to and from the Wireless Rether Server.
The problem of providing quality of service in wide area networks is a widely studied topic. However, not much work has been done to study and provide QoS in local area networks. Thus, there exists a need to develop a method and a system for meeting the quality of service needs of users in a local area network.